1. Field of Invention
This invention is directed to a machine for polishing semi-conductor wafers, and more particularly, to a machine in which the polishing rate of the wafer surface and the condition of the polishing pad may be continually monitored.
2. Description of the Prior Art
Machines for preparing and fabricating semi-conductor wafers are known in the art. Wafer preparation includes slicing semi-conductor crystals into thin sheets, and polishing the sliced wafers to free them of surface irregularities, that is, to achieve a planar surface. In wafer fabrication, devices such as integrated circuits or chips are imprinted on the prepared wafer. Each chip carries multiple thin layers of conducting metals, semiconductors and insulating materials such as oxides, each of which may require polishing during fabrication. The polishing process may be accomplished by an abrasive slurry lapping process in which a wafer mounted on a rotating carrier is brought into contact with a rotating polishing pad upon which is sprayed a slurry of insoluble abrasive particles suspended in a liquid. Material is removed from the wafer by the mechanical buffing action of the slurry. The polishing step often includes a chemical mechanical polishing ("CMP") process. CMP is the combination of mechanical and chemical abrasion, and may be performed with an acidic or basic slurry. Material is removed from the wafer due to both the mechanical buffing and the action of the acid or base.
Devices for performing chemical mechanical polishing are known in the art, for example, U.S. Pat. No. 5,308,438 to Cote et al, incorporated by reference. The device of Cote includes a rotatable circular polishing platen having a circular polishing pad mounted thereon. A rotatable polishing head or carrier adapted for holding and rotating a workpiece such as a semi-conductor wafer is suspended over the platen. The carrier and platens are rotated by separate motors. A slurry dispensing tube is disposed over the polishing pad. In operation, a slurry, for example, an oxidizing agent such as iron nitrate dispersed in water with aluminum oxide particles suspended therein, is dispensed on the upper surface of the rotating polishing pad. The rotating wafer is brought into contact with the pad and is polished due to the mechanically abrasive action of the aluminum oxide particles and the chemically abrasive action of the oxidizing agent.
Chip fabrication requires the formation of layers of material having relatively small thicknesses. For example, a typical metal conducting layer will have a thickness on the order of 2,000-6,000 .ANG., and a typical insulating oxide layer may have a thickness on the order of 4,000 .ANG.. The thicknesses will depend upon the function of the layer. A gate oxide layer may have a thickness of less than a hundred .ANG. and a field oxide layer may have a thickness of several thousand .ANG.. Nonetheless, the thickness of the layers must be formed within very strict tolerances, for example, 500 .ANG., in order to ensure that the desired operating parameters of the chip are achieved. Further, if the tolerances are not met, short circuits, or other defects which result in inoperative chips may result.
During chip fabrication, the layers are formed and then selected amounts of material must be removed without removing excess amounts of the underlying material in order to provide layers having thicknesses within the desired tolerances. One way to ensure that the selected amounts of material are removed in order to form layers having the desired thickness is by monitoring of the thickness of the layers during CMP. For example, the surface of the wafer may be physically examined by techniques which directly ascertain the dimensional and planar characteristics of the wafer, utilizing tools such as surface profilometers, ellipsometers or quartz crystal oscillators. However, use of these devices requires that the wafer be removed from the CMP apparatus. If the wafer does not meet specifications, it must be reloaded onto the apparatus and polished again. This process is time consuming and labor intensive, and decreases production efficiency. Further, if it is determined that too much material has been removed, the chip may have to be returned to the location at which the layers of material are applied, or may even be unusable.
A second way to ensure that desired amounts of material have been removed is by real time monitoring of the layer thickness as the wafer is being polished. Such a device is shown in U.S. Pat No. 5,240,552 to Yu et al, incorporated by reference, which makes use of acoustic waves which are directed at and reflected from the wafer during CMP. The reflected waves are detected and a determination is made of the total time the wave has travelled in the wafer. If the velocity of the waves through the water is known, the total thickness of the wafer and thus of a film layer thereon can be determined. However, the device of Yu et al requires complex circuitry for generating and detecting the acoustic wave. In particular, Yu makes use of a first piezoelectric transducer which is disposed between the wafer carrier and carrier pad in order to generate, in response to a voltage, an acoustic wave which will travel into and be reflected from a wafer, and a second similarly disposed piezoelectric transducer which converts the reflected acoustic wave back into a voltage. The transducers must be connected by conducting wires to an analyzing circuit, with the wires disposed in holes formed through the carrier. The fact that the Yu device includes components which are placed in physical contact with the moving parts of the CMP apparatus complicates the apparatus. Further, over time the effectiveness of the circuit may be deteriorated by the motion ;associated with polishing.
In principle, CMP also can be monitored by knowing with great certainty the CMP rate, that is, the rate at which material is being removed from the layer being polished. However, several factors affect the polishing rate. For example, during polishing the abrasive material of the slurry becomes embedded in the material of the polishing pad, reducing the density of material in the slurry. Further, the material being removed from the layer being polished becomes embedded in the pads, and the pads tend to become degraded over time during the polishing procedure. All of these factors affect the CMP rate and as a result, the rate is not constant throughout the usable life of a pad. In fact, the CMP rate is erratic even during CMP of one production lot of wafers and in some cases, CMP of a single wafer.
With reference to FIG. 1, a graph of the CMP rate versus total pad use time is shown for a single polishing pad. The graph of FIG. 1 is for oxide polishing with a slurry of Ceria (CeO.sub.2). The CMP rate of a layer of a CVD (chemical vapor deposition) oxide as expressed in the reduction of thickness of the layer per minute, is not stable, especially in the earlier stages of the usable life of a pad under approximately 600 minutes when the polishing rate increases greatly with continued use. Though the polishing rate is less erratic thereafter, it still varies above and below the rate of 7000 .ANG. per minute. Furthermore, it is not possible to tell when the pad has worn out, which would cause the polishing rate to be reduced.
Accordingly, due to the non-stable CMP rate, in order to ensure that selected amounts of material are removed within acceptable tolerances using knowledge of CMP rate, it is necessary to measure the polishing rate before each production lot of wafers, and in some cases, where the tolerances are small, before CMP of each wafer. Such measuring requires that at least one otherwise usable production wafer be monitored during a test polishing procedure to make an accurate determination of the polishing rate. Thus, both time and a usable wafer must be wasted. As a result, the overall efficiency of production during use of CMP is compromised. Further, even with these procedures, it is not possible to determine the instantaneous CMP rate.